Short answer is, you can't. Professional thieves won't be stopped by anything short of having armed people always on guard.
Now, non-professional thieves steal only what's easy, and leave alone what's hard to steal.
First thought: padlocks can be cut through. I'm more comfortable knowing that my horse is on a lot of space and it would be annoying to catch him; if a horse is stalled he's just there, ready, he just lacks a red ribbon. This is my opinion and I'm in no way saying that horses should not be stabled at night. I'm just saying that inside is not particularly safer than outside.
Oldest alarm system on earth: dogs. Small-thieves who just grab what's on hand won't risk being attacked. Plus if you train the dog to not eat anything that's outside their bowl.
Near the barn or shed, you can put a fake camera with a movement sensor and bright light; so when there's any movement, the light turns on and it looks like the camera is recording. Not many people want to steal in bright light. Or you can buy a real camera and record, but it's more expensive.
Be around. Don't have a routine. Make sure that, if a person is watching you, he can't really know when there will be some hours of "no one there", and when you'll come back. Thieves prefer to "work" when they know they won't be interrupted.